But it is not practicable to derive separate unit hydrograph for each possible time- intensity pattern. (v) It is commonly seen that no two rain storms have same pattern in space and time. Obviously the principle of time invariance is really valid only when the time and condition of the drainage basin are specified. (iv) The physical basin characteristics change with seasons, man-made structures in the basin, conditions of flow etc.

The reason is that the storm rainfall gets mixed up with the snow pack and may produce delayed runoff differently under different conditions of snow pack. (iii) Also when snow covered area in the drainage basin is significant the unit hydrograph method becomes inapplicable. (ii) The unit hydrograph method cannot be applied when appreciable portion of storm precipitation falls as snow because snow-melt runoff is governed mainly by temperature changes. The flood discharge at the basin outlet can then be estimated by combining the sub- basin floods adopting flood routing procedure. The catchment has to be divided into sub-basins and the unit hydrographs developed for each sub-basin. When the area of the drainage basin exceeds a few thousand km 2. Beyond it the reliability of the unit hydrograph method diminishes. The limiting size of the drainage basin is considered to be 5000 km 2.

Obviously bigger the area of the drainage basin lesser will be the chances of fulfilling the assumptions enunciated above. Also on large areas effective rainfall is very rarely uniform at all locations, within its specified duration. In practice, however, uniformly distributed effective rainfall rarely occurs on large areas. (i) In theory, the principle of unit hydrograph is applicable to a drainage basin of any size. This assumption is called principle of time invariance. In other words the hydrograph of direct runoff resulting from a given pattern of effective rainfall will remain invariable irrespective of its time of occurrence. (v) The hydrograph of runoff from a given drainage basin resulting, from a given pattern of rainfall reflects all the combined physical characteristics of the basin. This important assumption is called principle of linearity or proportionality or superposition. (iv) The ordinates of the direct runoff hydrographs having same time base (i.e., hydrographs due to effective rainfalls of different intensity but equal duration) are directly proportional to the total amount of direct runoff given by each hydrograph. (iii) The effective rainfalls of equal (unit) duration will produce hydrographs of direct runoff having same or constant time base.

Areal flood definition